1. Field of the Invention
The invention relates to a magnetic resonance apparatus, comprising a mainly cylindrical RF coil having a central axis which extends in its longitudinal direction, which coil has a symmetry plane which contains the central axis and a reference plane which extends perpendicularly to the symmetry plane and which also contains the central axis, and also comprises a number of rod conductors which extend, parallel to the central axis, across a mainly cylindrical surface and loop conductors which extend around the central axis near the ends of the rod conductors, the length of the rod conductors situated near the symmetry plane being greater than that of the rod conductors situated near the reference plane. The invention also relates to an RF coil suitable for use in such an apparatus.
2. Description of the Related Art
An example of such an apparatus is known from EP-A-0 141 383. FIG. 5D of the cited document shows an RF coil, one of the loop conductors of which is circular whereas the other loop conductor is more or less saddle-shaped, so that recesses are formed at one end of the coil. A magnetic resonance apparatus comprising an RF coil of this type is suitable for examination or imaging of the head of a patient by means of magnetic resonance. To this end, the coil can be arranged around the head of a patient, the shoulders of the patient fitting in the recesses so that the coil can enclose the head completely. Except for a window for observation of the face of the patient, provided at one side, the known coil has a four-fold cylindrical symmetry. One or more of the rod conductors are interrupted so as to form said window. The location of the window is chosen so that the interrupted rod conductors carry no or hardly any current during operation of the coil, so that the interruptions hardly influence the symmetry of the coil from an electrical point of view. The cited document explains that an RF coil having four-fold cylindrical symmetry offers a number of advantages, notably because such a coil has two orthogonal resonant modes at the same resonant frequency, enabling quadrature activation or read-out of the coil, both modes being simultaneously excited or received.
It has been found that there is a need for a magnetic resonance apparatus which is suitable for examination of the neck of a patient, that is to say a part of the body which extends from approximately 1 cm above the eyes as far as the third cervical vertebra. An apparatus comprising the known RF coil is not suitable for this purpose, because the coil is only capable of enclosing the head. This problem could in theory be solved by making the recesses for the shoulders deeper, but it has been found that in that case the signal-to-noise ratio of the coil is substantially degraded.
It is an object of the invention to provide a magnetic resonance apparatus of the kind set forth which is particularly suitable for examination of the neck of a patient with an acceptable signal-to-noise ratio. To achieve this, the device in accordance with the invention is characterized in that the rod conductors which are situated near the symmetry plane and to one side of the reference plane have a length which is greater than that of the rod conductors which are situated near the symmetry plane and to the other side of the reference plane.
The invention is based on the recognition of the fact that when the part of the known RF coil which faces the feet of the patient is simply made longer, an RF field will be generated not only at the back side of the patient (at the area of the cervical vertebrae), but also at the area of parts of the patient which need not be imaged, notably at the side of the breast. The latter field, therefore, does not contribute to the acquisition of usefull information concerning the body, but does contribute to the noise of the system, so that the signal-to-noise ratio is adversely affected. The RF coil of the device in accordance with the invention is shaped so that at one side it extends further in the direction of the feet of the patient than at the other side. When the coil is arranged around the head of the patient so that the longest rods are situated at the back side of the patient, these long rods will extend as far as underneath the third cervical vertebra, whereas the shorter rods at the side of the breast will extend only as far as the chin. As a result, a substantial part of the field generated at the breast side of the patient will be situated in the void between the chin and the body. This part of the field does not dissipate and does not contribute noise, so that the signal-to-noise ratio is enhanced. Moreover, as a result of this step the RF coil may be smaller (contain less conductor material), so that the coil losses (copper losses) are also reduced. Thus, again less noise is generated.
The signal-to-noise ratio can be further improved in a preferred embodiment of the device in accordance with the invention which is characterized in that the distance between rod conductors situated diametrically near the reference plane is smaller than the distance between rod conductors situated diametrically near the symmetry plane. This embodiment takes into account the fact that the cross-section of the head is approximately oval, resulting in voids between the coil and the sides of the head when an RF coil having a circular cross-section is used, so that the coil would become unnecessarily large, leading to the described adverse effects for the signal-to-noise ratio. The cross-section of the RF coil in said preferred embodiment is adapted substantially to that of the head.
A potential drawback of the device in accordance with the invention consists in that, due to the abandonment of four-fold symmetry, it is no longer ensured that the coil has two orthogonal resonant modes with the same resonant frequencies. This drawback can be eliminated by a further embodiment of the device in accordance with the invention which is characterized in that at least one capacitive element is connected in series with each rod conductor, the capacitance connected in series with each of the rod conductors near the symmetry plane being greater than the capacitance connected in series with each of the rod conductors near the reference plane. When the values of the capacitances connected in series with the rod conductors, being variable, if desired, are suitably chosen, an approximately equal contribution of the currents in the rods is achieved. As a result, over the major part of its length the coil is electrically equivalent to a coil having four-fold symmetry, so that quadrature read-out is readily possible.
This equivalence does not hold, or only to a small extent, for the end portion of the coil which faces the feet of the patient and which contains rod conductors only at the back side of the patient. Consequently, quadrature operation of the coil is less attractive, notably when measurement takes place in that portion of the RF coil. Therefore, a further embodiment of the device in accordance with the invention is characterized in that there are provided switching means for selectively deactivating the connection between one of the two connection means and the transmitter and/or receiver means. In this embodiment a change-over from the quadrature mode (where both orthogonal resonant modes are excited or received) to a linear mode (where only one resonant mode is active) is readily possible.